There is presently considerable interest in the use of fingerprint sensing for user identification or verification purposes. For this, a person's fingerprint is acquired by means of a fingerprint sensing device whose output is processed and compared with stored characteristical data of one or more fingerprints to determine whether a match exists. The most common form of fingerprint sensing device employs an optical imaging technique in which light from a light source is directed into one face of a glass prism, reflected from a second face and passes out from the third face where it is detected by a form of an electronic camera such as a CCD with associated optical elements. A finger is placed on the second face and where ridges of the fingerprint contact the glass face reflection no longer takes place. Through this effect an optical image of the fingerprint can be detected by the camera.
More recently, other forms of fingerprint sensing devices have been proposed which rely on different techniques for sensing fingerprint patterns, using for example alternative approaches such thermal sensing, pressure sensing, capacitive sensing, and skin resistance sensing. In these, an array of sensing elements define a sensing area over which a person's finger tip is directly placed and the need for an optical system is eliminated, which can result in a much more compact device.
There is currently much interest in incorporating fingerprint sensing devices in a wide range of products for security reasons. For example, the growth of the internet and the need for security when conducting internet commerce transactions has led to proposals for fingerprint sensing devices to be built into desk top and portable computer systems. Security of banking transactions using automatic teller machines can also be improved using fingerprint sensing. Similarly it has been proposed to use fingerprint sensing devices in other portable electronic equipment such as mobile phones and smart cards as a means of preventing unauthorised usage.
For many of these applications there is a need for a fingerprint sensing device which is both compact, particularly where portable electronic equipment is involved, and relatively inexpensive. The conventional optical image sensing device, requiring a light source, prism and image sensor, is both bulky and expensive, and therefore wholly unsuited to such uses. Of the other kinds of sensing devices, those using the capacitive sensing approach are considered to offer the most promise in view of their potential performance, low overall size, ease of manufacture and low cost. Typically, such fingerprint sensing device comprises an array of individual sensing elements, each including a sense electrode, over which a finger is placed and operate by sensing a measurable capacitive effect produced by the sense electrode in combination with an overlying ridge of the user's fingerprint.
Examples of capacitive fingerprint sensing devices are described in U.S. Pat. No. 5,325,442 (PHB 33628), PCT WO 97/40744 (PHB 34068) and PCT IB 98/00407 (PHB 34153). In these, each sensing element in the array consists of a metal sense electrode connected via one or more switching devices to sets of address conductors and the sensing elements are operated using an active matrix addressing scheme to provide a measure of the individual capacitances formed between each sense electrode and respective overlying ridge or valley of a person's fingerprint placed on a sensing surface provided by a layer of dielectric material overlying the sense electrodes and on whose surface the finger is placed. The device can be in the form of a semiconductor integrated circuit, using a silicon wafer, or a thin film device in which the sense elements and address conductors comprise thin film circuit elements carried on an insulating substrate such as glass and formed by depositing and patterning thin film insulator and metal layers and amorphous or polycrystalline semiconductor compounds for the switching devices, e.g. diodes or thin film transistors. Another example of a capacitive type fingerprint sensing array produced in the form of an integrated chip is described in EP-A-0790479. In this device each sensing element consists of two laterally--spaced sense electrodes connected to an amplifier circuit.
Although these devices are capable of satisfying some of the demands, particularly as regards compactness and manufacturing cost, there is a need for improvement. One problem which could be experienced in use is that of attempted fraudulent operation. It is conceivable, for example that a plastic replica of a person's finger, having surface topography, could be used to "fool" the device. Accordingly, it would be advantageous to be able to ensure that the finger whose print is being read is a "live" finger. It would be desirable also to improve the capabilities of the sensing device further with regard for example to their ease of deployment, and suitability for use, in small portable products such as mobile phones, personal digital assistants and the like, and their ability to function in combination with other components.